1. Field of the Invention
The present invention relates to an excimer laser generator used to process various materials such as metal, resin, glass, ceramics, or semiconductor, or used to assist a chemical reaction. More particularly, the present invention relates to an excimer laser generator capable of generating a laser beam which is stable in its energy and shape and having a long life. The present invention also relates to a blower and a heat exchanger for use in such an excimer laser generator. Furthermore, the present invention relates to a step-and-repeat exposure apparatus using such an excimer laser generator.
2. Description of the Related Art
An excimer laser generating system is the only laser generator that can generate a high output laser beam with a wavelength in an ultraviolet region, and is expected to be used in a wide variety of applications such as electronic and chemical industries and energy industries.
Herein the "excimer laser generator" or "excimer laser generating system" refers to a system for generating an excimer laser beam. In an excimer laser generating system, a laser gas such as Ar, Kr, Xe, KrF, or ArF filled in a manifold is excited by means of discharging or electron beam radiation. Excited atoms are combined with atoms in the ground state, thus creating molecules which can exist only in excited states. These molecules are called excimers. Since the excimers are unstable, they immediately emit ultraviolet light and fall to the ground state. This transition is called bond-free transition. In excimer lasers, the ultraviolet light generated by the bond-free transition is amplified by an optical resonator composed of a pair of mirrors. The amplified light is emitted to the outside as a laser beam.
With reference to FIG. 1, an excimer laser generating system disclosed in U.S. Pat. No. 5,029,177 is described below.
As shown in FIG. 1, the excimer laser generating system includes: a laser chamber 1; an oscillation spectrum narrowing module 2 for controlling the oscillation line width; a high-voltage pulse power module 3; an optical monitor module 4 for detecting the energy intensity and the wavelength of the laser output; a high-voltage power supply 5; and a controller 6 for controlling the entire parts of the excimer laser generating system.
When the excimer laser generating system is used as an exposure light source of a step-and-repeat exposure apparatus for producing an electron device, the computer controller 6 is connected via an interface 7 to a control unit 8 and a computer 9 of the step-and-repeat exposure apparatus.
The laser chamber 1 is filled with a gas such as F.sub.2, Kr, or Ne. A high-voltage pulse generated by the pulse power module 3 is applied to the cathode electrode in the laser chamber so that a discharge occurs in the gas, which causes light emission. The light is repeatedly reflected between an output mirror 10 and an optical system in the spectrum narrowing module 2, and is amplified during the travel. The amplified light is emitted to the outside.
To obtain a laser beam having a desired wavelength, a part of the emitted laser beam is reflected by a half-mirror 11 and directed to the optical monitor module 4. The optical monitor module 4 measures the wavelength and other characteristics of the light. In response to the measurement result provided by the optical monitor module 4, a stepping motor 12 is driven so that the optical system of the spectrum narrowing module 2 is optimized. The optical power of the laser beam is also monitored by the optical monitor module 4.
FIG. 2 is a cross-sectional view of the laser chamber 1, taken in a direction perpendicular to the direction of laser oscillation.
As shown in FIG. 2, the laser chamber 1 has two aluminum housing members 13 and 14 which are combined together via an O-ring 15 so as to form a sealed chamber. A cathode electrode 18 is fixed to the chamber 1 via a cathode electrode supporting member 17 and an insulator 16. Anode electrode 19 is fixed to the housing member 13 via an anode electrode supporting member 20. The laser chamber 1 further includes a connector 21 for connecting the cathode electrode 18 to the pulse generator 3. There is also provided a sealing member such as an O-ring 22.
Inside the laser chamber 1, there is a blower 23 for circulating gas within the chamber 1. A heat exchanger including a heat-transfer pipe is also provided in the chamber 1 to cool the gas heated by discharging. The laser chamber 1, the blower 23, and the heat exchanger 24 are made of stainless steel or an aluminum alloy wherein the outer surfaces of these elements are usually electro-polished or bright-annealed. An air filter 25 is fixed to the housing so that gas is introduced into the filter via a gas inlet 26 thereby removing particles such as metal fluoride particles produced by the reaction of the electrode metal with fluorine gas.
Referring to FIG. 3, a gas supplying system which has been proposed previously by the inventor of the present invention will be described below. This gas supplying system has been designed to properly supply gas into the laser chamber 1.
In the specific example shown in FIG. 3, there are three gas supply lines: a 1% Kr/Ne gas supply line 31, a 1% F.sub.2 /1% Kr/Ne gas supply line 32; and a He gas supplying line 33. The respective gas supply lines have orifices 34, 35, 36 for controlling the flow rates and also have valves 37, 38, and 39 connected to a manifold 40. He gas is used as a purging gas when window plates of the laser chamber are replaced with new ones.
A plurality of gas supply pipes are connected to the manifold 40. Of these gas supply pipes, a gas supplying connection pipe 41 has a chamber valve 42 and a valve 43. The manifold 40 is also connected to a F.sub.2 gas supply pipe 44 which is connected to the connection pipe 41 via an injection valve 45 and a flow rate control orifice 46.
The gas supplying system further includes: a spring valve 47 which is opened when the pressure in the manifold 40 increases to an abnormally high level greater than an allowable level; an exhaust line 48; a valve 49 which is operated by hand as the valve 43 to exhaust the gas when a trouble occurs inside the laser chamber; F.sub.2 gas safening treatment equipment 50; a vacuum pump 51; and a pressure gauge 52.
When it is desirable to fill the laser chamber with a laser discharging gas containing 0.1% F.sub.2, the laser discharging gas can be introduced into the chamber as follows. First, 1% F.sub.2 /1% Kr/Ne gas is introduced, while monitoring its pressure, into the chamber 53 via the manifold 40 and the pipe 41 until the pressure reaches 30 kPa. Then 1% Kr/Ne gas is introduced until the pressure reaches 300 kPa so that the chamber is filled with the mixed gas containing 0.1% F.sub.2.
The F.sub.2 concentration decreases with the repetition of laser oscillation. This causes a reduction in the optical output power of the laser. Therefore, each time the optical output power of the laser drops to a level lower than a predetermined value, it is required to supply a proper amount of 1% F.sub.2 /1% Kr/Ne gas into the chamber 53 via the manifold 40 and the pipe 44.
The conventional excimer laser generating system has the following problems which were not known in the art before the inventor of the present invention found them.
A first problem in the conventional excimer laser generating system is that the F.sub.2 gas has to be supplied as frequently as every 5.times.10.sup.6 pulses to obtain required optical output power.
A second problem is that the reaction produces fluoride which absorbs laser light. This makes it impossible to obtain stable optical power. For example, the formation of fluoride can cause a variation in the optical output power as large as 5% to 10%. To remove the above fluoride, the entire gas in the chamber 1 must be replaced with a new gas every 5.times.10.sup.7 to 1.times.10.sup.8 pulses.
A third problem is that some fluoride produced by chemical reaction in the chamber deposits on a window of the laser chamber. As a result, the window must be replaced every 1.times.10.sup.9 pulses.
A fourth problem is that great degradation occurs in the electrodes 18 and 19 in the laser chamber, and thus these electrodes must be replaced every 3.times.10.sup.9 pulses.
As described above, the operation life of the conventional excimer laser generating system is very short.
In addition to the above problems of short life, the conventional excimer laser generating system also has the following problems to be solved.
A fifth problem is that the accuracy of the pressure gauge for monitoring the gas pressure is not high enough to control the mixing ratio of the gas introduced into the laser chamber. This results in poor controllability of the optical output power of the laser.
A sixth problem is that the inside of any of the gas supply pipes is exposed to atmosphere, which can occur when a gas source of the gas supplying system is replaced, a small amount of water is adsorbed on the inner wall of the pipe and it is difficult to perfectly remove it by a common outgassing technique. When F.sub.2 gas is introduced into the gas supply pipe, the F.sub.2 gas can react with the adsorbed water, which makes it difficult to precisely supply a desired amount of F.sub.2 into the laser chamber. The contamination of water in the laser chamber results in degradation of the chamber and thus results in a reduction in life.
The above problems have been found by the inventors of the present invention during their activity of developing an excimer laser generating system capable of stably outputting optical power and having a long life. The present invention has been achieved by solving these problems.